Moisture determining apparatus



De@ 16, 1941- H. w. BARRE-w 2,2%;@4

MOISTURE DETERMINING APPARATUS Filed Nov. 2, 1938 A TTORNEY Patented Dec. 16, 1941 MOISTURE DETERMINING APPARATUS Harold W. Bartlett, Berkeley, Calif., assignor to Rosenberg Bros.

& Co., San Francisco, Calif.,

a corporation of California Application November 2, 1938, Serial No. 238,383

Claims.

This application relates generally to apparatus for determining moisture content of various materials. In practice the invention has been successfully applied for accurate determination of moisture content of grains, like wheat, rice, oats, etc., and it can also be applied to a variety of food products and other material, including for example tobacco, various vegetables, dried fruits, like raisins, prunes and the like, nuts and other materials Where it is desired to secure a rapid and relatively accurate determination of moisture content without a specific moisture analysis.

In the past it has been known that the moisture content of various materials results in a change in electrical conductivity. Apparatus making use of this principle has met with some degree of success, particularly where no disturbing factors are involved, as for example the presence of water which is not evenly distributed throughout the mass. Where certain disturbing factors are involved, such as unequal distribution of moisture in the mass, moisture determination by electrical conductivity is inaccurate and cannot be relied upon. Furthermore, such apparatus can only be used with accuracy for a limited range of moisture content, and considerable care must be taken to maintain a sample under proper predetermined conditions of pressure in order to secure the desired results.

As an alternative to indicating the conductivity or resistance of a sample, it has also been proposed to measure moisture content conductivity by electrical apparatus serving to indicate the dielectric capacity of a given sample. The dielectric capacity or specific conductive capacity of various materials varies as the'moisture content varies. However, apparatus based upon this principle such as has 'been used in the past has not been suflciently critical for accurate moisture determination, and it has been applicable to a wide range of moisture content. According to `my observations this is largely due to the effect of the resistance component oiered by the sample upon the power factor of the electrical energy impressed upon the sample by the testing apparatus.

It is an object of the present invention to provide a moisture determining apparatus which operates by measuring dielectric capacity of a sample, and which will give accurate results for a wide range of moisture content. For example, the present apparatus will operate successfully for a range of moisture content from substantially zero to one hundred percent. Thus the present invention is applicable to both liquids and solids.

A further object of the invention is to provide apparatus of the above character which will not be detrimentally affected by extraneous factors such as voltage variations upon the current supply line, and which will afford a positive and highly critical indication.

Additional objects of the invention will appear from the following description in which the preferred embodiment of the invention has been set forth in detail in conjunction with the accompanying drawing.

Referring to the drawing:

Fig. 1 is a circuit diagram illustrating diagrammatically apparatus in accordance with the present invention.

Fig. 2 is a. side elevation in cross section, showing a sampling device for use in connection with the present apparatus.

The apparatus as illustrated in the drawing makes use of a sample cell including a pair of spaced electrodes between which a sample of material may be placed. This cell is coupled to a high frequency electrical oscillator generator in such a manner that the capacitance afforded by the cell forms a capacitive element of a resonant circuit which in turn determines the frequency of operation of the oscillation generator. The resistance component afforded by the sample in the cell is effectively neutralized by virtue of the fact that the oscillation generator alfords a negative resistance component.

Referring rst to Fig. 1 of the drawing, the apparatus illustrated includes the cell I0 for retaining the sample to be tested, and which is coupled to the electrical net work II. The electrical net Work includes the electron relays or vacuum tubes I2 and I3, which are arranged to generate electrical oscillations, and the tubes I4 and I5, which are arranged to stabilize the frequency of oscillations generated. As will be presently explained connections to tubes I2 and I3 are preferably such that they operate as dynatron oscillators. A dynatron oscillator of the electron relay type is one in which the plate is maintained at a lower potential than the screen grid and which employs the phenomenon of secondary emission to obtain a negative resistance.

The cell I0 as illustrated diagrammatically in Fig. 1 includes the electrodes I1 and I8. Spaced plates can be utilized having their surfaces covered with suitable insulating material, and between which the sample of material may be placed. As illustrated diagrammatically in Fig. l and as will be presently described with reference to Fig. 2, it is preferable that one electrode, as for example electrode I'I, be in the form of a tube or rod disposed axially of the outer tubular electrode I8. The outer electrode I8 is connected to ground I9, thereby tending to make operation of the cell immune with respect to surrounding objects such as the hand of the operator.

The tube I2 is preferably a tetrode having its plate 2| connected to one side of a resonant circuit which includes the inductance 22, and shunt condensers 23, 24 and 25. A switch 26 is shown for disconnecting condenser 25. The other side of inductance 22 connects to ground 28 through the high frequency by-pass condenser 29. A suitable source of plate potential is connected to the plate of relay I2, as for example a rectifier and filter 3| which can be supplied with alternating current. The positive lead from the rectier and filter 3I- is shown connected to the conductor 32, which in turn connects to the low potential side of inductance 22 through the resistor 33. The other end of this resistor is grounded. The screen 34 of relay I2 is connected directly to the lead 32 byconductor 36.

The grid or control element 31 of tube I2 connects with a stabilizing relay or vacuum tube I4. Thus grid 31 is shown connected to one side of the inductance 38, which is inductively coupled to the inductance 22. The other side of inductance 38 connects with the grid 39 of the relay I4. Cathode 4I of electron relay I4 together with cathode 42 of relay I2, connect to a common lead 43, which in turn is connected to ground through the high frequency by-pass condenser 44.

The electron relay or vacuum tube I3 is likewise shown as a tetrode, and is connected to circuits in such a manner as to form a dynatron oscillator. The plate 2| of tube I3 connects to one side of the inductance 46, which is shunted by -the variable condenser 41. The other side of inductance 46 is directly connected to the corresponding side of inductance 22, by conductor 48. Likewise, the same side of inductance 46 is connected to ground 49 through the high frequency by-pass condenser 5I. The screen 34 of electron tube I3 conductively connects to the lead 32, as with the tube I2. Likewise the grid 31 is grounded and is connected to one side of an inductance 32, which is inductively coupled with inductance 46 and has its other side connected with the grid 39 of tube l5. The cathodes 4I and 42 of tubes I5 and I3 connect to the common conductor 54, which in turn connects to groundthrough the high frequency by-pass condenser 55.

The negative side of the rectifier and filter 3| is grounded as indicated. 'Ihe cathode conductors 43 and 54 connect to the biasing resistors 51 and 58 which in turn connect to ground 59. The plates 60 of the electron relays I4 and 5 both connect through resistors 6I and E2 to the supply lead 32.

As a convenient and effective type of indicating means, I have shown a Crooks tube type of visual indicating device 63, such as are used to indicate the tuning of radio broadcast receivers. The heated cathode 64 of this device is shown connected to ground through the resistor 66. 'Ihe plate or anode 61 is directly connected to the supply lead 32. The screen 68 connects with plate 61 through the resistor 69. The control grid or element 1I is coupled to both the plates 60 of tubes I4 and i5, through the condensers 12 and 13. Also grid 1I is connected to ground through resistor 10. Device 33 affords an illuminated screen with a variable iris which indicates the additive potential obtained from the plate circuits of both the tubes I4 and I5. Also when two oscillation generators are operating at nearly the same frequency, whereby the beat or differential frequency is relatively low, as for example cycles or less per second, this beat frequency is clearly indicated.

It will be evident that the capacitance afforded by the cell I0 will be determined in part by the dielectric value of the material placed between the electrodes I1 and I8. Likewise for a given sample the cell as a whole assumes a given value of capacitance, which becomes a reactive element of the resonant circuit which includes the inductance 2 2, and which controls the frequency of oscillations generated. Use of three condensers 23, 24 and 25 facilitates calibration and adjustment of the apparatus. In a typical instance where the samples range from zero to twentyve percent moisture, condenser 23 may range from 0 to 25 mmf., condenser 24 from 0 to 100 mmf., and condenser 25 from 0 to 25 mmf. Condenser 25 is set at such a value that when the cell I0 is connected in the circuit but is empty, with the condenser 24 set at 50% of full value, the oscillations generated by tube I2 are absolutely in synchronism with the setting of oscil- `lation generator I3. For the values just given for condensers 23, 24 and 25, condenser 41 across inductance 46 can afford a maximum capacitance of 150 mmf., and is adjusted to afford a given set frequency of operation, as can be determined by suitable Calibrating instruments.

Before proceeding with an explanation of the apparatus, it can be pointed out that the frequency of operation of the tubes I2 and I3 is stabilized by electron relays I4 and I5. In this connection it will be noted that the bias resistor 51 for tube I4 is also the bias ristor for the tube I2. Thus any increase in excitation of the tube I4, resulting from an increase in self excitation of the dynatron formed by tube I2, causes an increased current flow through the common biasing resistor 51 with the result that there is a tendency for the bias upon the grid or tube I2 to increase to suppress self excitation. Therefore the frequency of operation of the dynatrons and the amplitude of the potentials repeated by the tubes I4 and I5 remain substantially constant. Also, although the dynatrons and the relays I4 and I5 are in push-pull relationship, there is no tendency for one dynatron to pull the other generator into synchronism.

Assuming that the apparatus has been properly adjusted and that one has the necessary calibration charts for interpreting readings obtained, a given sample is placed within the cell I0. With the network I I in operation, the operator adjusts the variable condenser 24 and observes the indicating device 63, until this indieating device shows a beat frequency of minimum frequency and maximum amplitude. This means that the two dynatrons are operating at substantially the same frequency. 'I'he setting of condenser 24 can now be accurately translated itself. For example, where a sample affords a resistance of 4000 ohms, the negative resistance component afforded by the tube I2 should not be more than 4000 ohms. For samples containing more than say 25% moisture, it may be necessary to use two tubes operating in parallel, in order to secure a required negative resistance component of the required value. The negative resistance component makes the apparatus immune to power factor losses such as would make ordinary apparatus inoperative or inaccurate for samples of relatively high moisture content.

In a typical apparatus operating as described above, for measuring samples varying from to 25% moisture, including both solids and liquids, the frequency of operation of the dynatron tube i3 should be relatively high in order to secure the desired critical indication, as for example in the neighborhood of from seven to eight hundred thousand cycles. Portions of the network should be suitably shielded as is well known to those skilled in the art, in order that extraneous objects may not aiIect the frequency of operation or the accuracy of the results obtained. In actual operations I have found it practical to maintain the frequency of operation sufllciently constant so that the indicating device 63 indicates a beat frequency as low as one cycle per second.

Fig. 2 illustrates suitable structural details for the sampling cell. In this instance the inner electrode l1 is in the form of a metal tube, havlarger metal tube, having its axis coincident with the axis of electrode I1, and having its inner surface covered by the glass tube 11. The lower ends of both electrodes are attached to the common bottom wall or base 18, which carriesa central contact stud 19 connected to the electrode I1.

'I'he electrical network Il previously described is generally enclosed within a suitable cabinet, with one wall of this cabinet carrying provision for connection with the cell I0. Fig. 2 illustrates one top wall of such a cabinet formed of a metal panel 8l which is grounded and which carries insulating cup I2. A quantity of mercury 83 iscarried within cup 82 and connects with the terminal B4. The upper side of the cup 82 is apertured to receive the contact stud 19, substantially in the manner illustrated. The lower edge of the outer electrode Il is shown provided with depended buttons or studs 8l, for making connection with the grounded metal panel Il. Thus the outer electrode Il is connected to ground, while the inner electrode I1 is connected to the mercury in cup I2, which in turn makes connection with the electrical network. Il as illustrated in Fig. 1. In using the apparatus the vcup 82 can receive a predetermined volume of the material, or if desired the cup can be always filled with the material, and then this amount of material accurately weighed after making a test and both the weight and the volume taken into account in determining moisture content.

The insulating covering 1l and 11 for the electrodes can be omitted, provided one inserts a suitable blocking condenser in series with the cup.

I claim:

1.l In moisture testing apparatus, a sample cell including a pair of spaced electrodes between which a sample of material may be placed, a dynatron oscillator having a resonant circuit including said cell as a capacitive element of the same, electron relay means for stabilizing the frequency of operation of the dynatron, and means for indicating a given frequency of operation of the dynatron.

2. A moisture testing apparatus, a sample cell including a pair of spaced electrodes between capacitance of said cell for y Pair '0f dynatron oscillators,` one which a sample of material may be placed, a pair of dynatron oscillators, one of said dynatrons operating at a fixed frequency and the second having a resonant circuit including said cell as a capacitive element of the same, said resonant circuit also including a variable reactance whereby the dynatron associated with the same can be tuned by variation of said reactance to a condition in which it generates oscillations at the same frequency as the rst dynatron, separate electron relay stabilizing means associated with said dynatrons and serving to stabilize the frequencies of operation of the same, and indicating means coupled to receive electrical` oscillations from both said dynatrons and serving to indicate a critical condition in which both said dynatrons operate at the same frequency.

3. In moisture testing apparatus, a sample cell including a pair of spaced electrodes between which a sample of material may be placed, an electrical oscillation generator having negative resistance characteristics, said generator having a resonance circuit including said cell as a capacitative element of the same, whereby variations in the capacitance of said cell for dierent samples placed within the same aects the frequency of operation of the oscillation generator, means for stabilizing the frequency of operation of the oscillation generator, and means for indicating a given frequency of operation of the oscillation generator.

4. In moisture testing apparatus, a sample cell including a pair of spaced electrodes between which a sample of material to be tested may be placed, a pair of electrical oscillation generators each having negative resistance characteristics, one being capable of operation at a fixed frequency, and the second having a resonance circuit including said cell as a capacitative element of the same whereby a change in the different samples of material affects the frequency of operation of the second oscillation generator, means for stabilizing the frequency of operation of both said oscillation generators, and indicating means coupled with both of said oscillation generators and sewing to indicate generation of oscillations of substantially the same frequency by both said generators.

5. In moisture. testing apparatus, including a pair of spaced electrodesbetvireen which a sample of material may placed, a

o said dynatrons operating at a fixed frequency and the second havingl a resonance circuit including said cell as a capacitative element of the same, said resonance circuit also including a variable reactance whereby the' dynatrons associated with the same can be tuned by variation of said reactance to a condition in which it generates oscillations at the same frequencyas the first dynatron, separate electron relay means associated with said dynatrons and serving to stabilize the frequencies of operation of the same, and indicating means coupled to the outputs of said stabilizing means to receive electrical oscillations from both said dynatrons, said indicateacondition asample cell 

